In the manufacture of sulphuric acid by the contact process, large quantities of heat are generated and removed partly by cooling recirculating streams of concentrated acid ranging in strength from 93 to 99.5% sulphuric acid. In modern sulphuric acid plants, shell and tube heat exchangers are commonly used which are fabricated of stainless steel and anodically protected on the acid side to minimize corrosive attack on the stainless steel. In such coolers, the acid to be cooled is passed through the shell space and cooling fluid, typically water, is passed through the tubes. The cooling water is the dirtier of the two fluids and in most duties, cleaning is needed only on the water side. Other reasons for the acid circulating in the shell include easier anodic protection and better overall heat transfer coefficients, which allow of smaller acid coolers and, hence, lower costs.
Anodic protection is a technique applicable to metals, such as tantalum, aluminium, carbon steel and the stainless steels, which normally form a stable oxide film on the surface of the metal. In many environments, such films may be either unstable or not formed due to the nature of the liquid in contact with the metal. Anodic protection causes a current to flow across the metal surface such that an oxidizing condition is created leading to formation of the oxide film which is relatively insulating and protects the surface against the liquid medium. Thus, anodic protection can be used for those duties in which metal without anodic protection would dissolve rapidly, as well as in conditions in which the protection decreases the corrosive attack by several orders of magnitude.
Anodic protection was initially introduced in the 1950's and 1960's to protect carbon steel in an environment in which the metal would have dissolved in days or even hours without anodic protection against hot acid. Subsequently, the technique was used in the shell space of shell and tube exchangers to protect exposed stainless steel against corrosion by hot, concentrated sulphuric acid. The exchangers were designed, however, such that a significant cooler life was possible without anodic protection in the event that a short outage of the anodic protection system would not have catastrophic consequences.
To protect the shell space of a shell and tube heat exchanger, two types of cathodes are normally used. These are longitudinal cathodes arranged in the shell parallel to the tubes, and pin cathodes inserted in the acid inlet and outlet nozzles. Reference electrodes are needed to ensure that the appropriate degree of anodic protection is being supplied. In all cases, the cathodes must be insulated from the metal surface being protected and this is done by use of fluoropolymer sleeves or sheaths in the case of longitudinal cathodes and pin cathodes or glass in the case of the reference electrodes. Power requirements for anodic protection are quite small, for example, a large exchanger with 6000 square feet of heat transfer surface can be protected in most cases with a current flow of less than 20 amperes at a voltage of less than 1.5 volts, corresponding to less than 30 watts. Annual power consumption in such systems is therefore trivial in comparison to the capital cost of a cooler, which can range up to $500,000.
Longitudinal cathodes in such heat exchangers are normally made of proprietary alloys, such as Hastalloy B or C, and are arranged either in the bundle or in dome spaces on the exchanger if such spaces are available. The cathodes are inserted through an end of the exchanger and generally pass in a cathode tube through the water box and then through the tube sheet into the shell space to the opposite tube sheet. In some cases, the cathodes may pass through both tube sheets and both water boxes so that power can be fed to both ends of the cathode rod. Typically, the cathode diameter ranges from 1 cm to 1.5 cm.
To isolate the cathode electrically from the surface of the tubes, baffles, and shell being protected, the cathode is contained in an acid resistant sheath generally formed of a fluoropolymer, e.g. TEFLON.RTM. polytetrafluoroethylene. The sheath is, typically, perforated in the regions between and remote from baffles and solid near the baffles and tube sheets. In this way the possibility of current flow from the cathode direct to the exchanger metal is avoided. In practice, it has been found necessary to keep the cathode to metal gap at least 25 mm in size to avoid current short-circuiting. Similar sleeves are used in the seals on the ends of the cathode in the cathode tube where the cathode is extended to atmosphere with an air to acid seal, while around the cathode tube a water to air seal is provided.
Surfaces of, for example, stainless steel in hot concentrated acid in turbulent conditions are not automatically passive and power has to be applied to create the passive film. Initially, the surface will be partly passive and partly active with the active portion of the surface moving from one location to another at random. As current is applied, corrosion initially increases to form the anodic film. There is a maximum current needed which is referred to as the `critical current`. Clearly, if the current available is small, the ability to modify a large surface will be small and the possibility exists that the current may actually add to corrosion and not protect the surface. The size of the critical current will depend on the size of the unit to be protected, the past history of the metal surface to be protected, the fluid with which it is in contact, and the temperature of the surface. A more dilute acid such as 93%, which is used for drying of gases, will have a higher critical current than 98.5% acid, which is used for absorption purposes. Similarly, hot acid will be associated with higher critical current than cold acid.
The anodic protection phenomenon is also not just simply a matter of creating an anodic surface on the metal by application of an appropriate voltage. Excessive voltage can cause significant damage to the surface by a phenomenon known as `transpassive corrosion`. The oxide film on stainless steel depends on the voltage applied and as the voltage rises the relatively insulating and non-corrodable film changes and becomes porous, allowing metal to dissolve and be carried away into the acid. Transpassive corrosion by this mechanism can cause significant damage to the metal surface and such corrosion has been observed where a metal surface was exposed very close to a cathode supplying the protecting current. The voltage levels at which transpassive corrosion can occur are dependent on the same factors which affect the passive film.
The region of applied potential in which the passive film exists is known as the `passive` zone and varies in width with acid strength and temperature and narrows as the acid temperature rises or the acid concentration drops. Similarly, the boundary zone for transpassive corrosion moves lower at the same time, reducing the zone of safe protection of the heat exchanger at higher temperatures or lower concentrations.
Distribution of an appropriate protecting current throughout the shell space of an exchanger, which space may be as long as 13 meters with a diameter as wide as 1.3 m cannot be taken for granted. Often the protective voltage and current may be adequate at one end of the exchanger but not at the other. In such cases, trim cathodes are now used which consist of short sections of rod inserted at 90 degrees to the acid flow in the inlet or outlet acid nozzles of the exchanger. These cathodes have fluoropolymer sleeves to isolate them from the surface being protected and the extent of exposure of cathode surface or the resistance of the lines feeding current to the trim cathode surface can be varied to suit the circumstances. The power input from pin cathodes modifies the potential available in the region in which it is installed and can cause an appropriate reading on the reference electrode located in the same zone.
Present practice in such anodically protected coolers is to use longitudinal cathodes extending essentially the length of the tube bundle with current feed from one end and to use pin cathodes to supply additional current to allow the reference electrode readings to be within the control parameters at both ends of the cooler. In some cases electrical resistances have also been used in series with the cathodes and some coolers have power supplied to both ends of the cathode rods, as described, for example, in U.S. Pat. No. 4,588,022 to Sanz, issued May 13, 1986. Cathodes have also been located either in the dome space or in the tube bundle, with a clearance space around the cathode to avoid transpassive attack on adjacent tubes.
There are a number of features of construction and operation of existing anodic protection systems which are less than ideal.
A typical exchanger may have as many as twenty or thirty baffles. The effect of a pin cathode on current flow is limited by the baffle to the inlet or outlet pass where the cathode and reference electrode are located. Additional protection provided by a pin cathode therefore has little effect in the next baffle opening and corrosion may occur there without alarming the reference electrode.
The fluoropolymer sheath around the cathode rod is known to deform over time with temperature. Where the sheaths pass through baffles it is possible for deformation to be such that the cathode rod and sheath cannot be withdrawn through the baffle for maintenance without significant force which can damage the exchanger and in some cases withdrawal has been impossible.
Reduction in the thickness of the cathode rod, which is gradually corroding, is usually from the hot end and can then limit severely the current entering the exchanger.
The resistance of the cathode rod can cause excessive voltages at one end of the rod without generating adequate protection at the opposite end of the exchanger. This problem is especially severe on start-up of a cooler when a large unpassivated surface exists.
The use of holes in the cathode sheath to allow current to flow from the cathode to the acid results in restriction in current flow, which allows significant voltage differences to develop which can cause other cathodic processes to occur with formation of insoluble sulphate or sulphur deposits. These deposits can plug the holes in the sheath and insulate it from the acid. The higher the current density leaving the cathode, the higher the voltage required and the greater the possibility of corrosion and the formation of undesirable deposits.
In existing designs there is a need to cope with significantly different temperatures and current requirements at opposite ends of the acid cooler while the same cathode arrangements are used.
Accordingly, there is, therefore, a need for an anodic protection system which provides improved corrosion resistance to a heat exchanger.